Fixed compiler warning with possible uninitialized variable 'nid'.

  CC [M]  /home/mranostay/git/alsa-driver/pci/hda/patch_sigmatel.o
/home/mranostay/git/alsa-driver/pci/hda/../../alsa-kernel/pci/hda/patch_sigmatel.c: In function
‘stac92xx_parse_auto_config’:
/home/mranostay/git/alsa-driver/pci/hda/../../alsa-kernel/pci/hda/patch_sigmatel.c:2815: warning: ‘nid’ may be used
uninitialized in this function

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>